The present invention relates to an ammonia gas analyzer in which a sample gas containing NH.sub.3 is fed to a measuring channel where NH.sub.3 is converted into NO by a NH.sub.3 /NO converter and to a comparison channel where the concentration of NH.sub.3 in the sample gas is measured with reference to an increase in the amount of NO.sub.2 in the measuring channel. For example, combustion of fossil fuel results in large amounts of nitrogen oxides (NO.sub.x) and/or sulfur trioxide (SO.sub.3) which reacts with water to form sulfuric acid mist.
In order to exclude nitrogen oxides, for example, from the combustion exhaust gas, a nitrogen removal system utilizing a selective ammonia reduction method has been commonly used. In this nitrogen removing system, it is necessary to add an excess amount of ammonia to improve the efficiency of the nitrogen removing reaction of ammonia (NH.sub.3) with nitrogen oxides. As a result of this, non-reacted ammonia is left at an outlet of the nitrogen removing apparatus and it is exhausted through a chimney.
Recently, the exhaust of ammonia has been recognized as a source of the pollution due mainly to its disagreeable odor and the ammonia content of industrial discharges has been subjected to regulation. Therefore, it is necessary to measure the concentration of ammonia in the exhaust gas continuously and accurately.
FIG. 1 shows schematically a conventional ammonia gas analyzer for measuring ammonia concentration using an infrared NO gas analyzer. In FIG. 1, a gas sampler 10 is inserted in a chimney through which a combination of exhaust gases flows. The gas sampler 10 includes a filter 11, a gas sampling tube 12 connected to the filter 11, a protective sheath 13 for protecting the filter and the sampling tube and a flange 14 for mounting the gas sampler 10 on a chimney wall.
Combustion exhaust gas (referred to as "sample gas" hereinafter) collected by the gas sampler 10 is fed to a measuring gas channel 20 and to a comparison gas channel 30 branching at the inlet of the channel 20. In the measuring gas channel 20, a NH.sub.3 /NO converter 21 for converting NH.sub.3 contained in the sample gas into nitrogen monoxide (NO), a NO.sub.2 /NO converter 22 for converting nitrogen dioxide (NO.sub.2) converted eventually from a portion of the NH.sub.3 by the NH.sub.3 /NO converter 21 and contained in a gas therefrom into nitrogen monoxide (NO), a pump 23, a desiccator 24 and a filter 25 are disposed. A pump 33, a desiccator 34 and filter 35 are disposed in comparison channel 30. A NO.sub.2 /NO converter similar to the converter 22 should be included also in the comparison channel 30 if the exhaust gas itself contains NO.sub.2. If there is no NO.sub.2 in the exhaust gas and the NH.sub.3 /NO converter can convert it into NO completely, there is no need of providing the NO.sub.2 /NO converter 22 in the measuring gas channel 20.
The sample gas in the measuring gas system 20 is conducted to a measuring tank 43 of an infrared NO gas analyzer 40. The sample gas introduced to the comparison channel 30 is introduced into a comparison tank 44 provided in the channel 30. The infrared NO gas analyzer 40 includes a light source 41, a chopper 47 which is rotated by a motor 46 for chopping light from the light source 41, an optical separator 42 for separating the light from the light source into two light beams, the measuring tank 43, the comparison tank 44 and a detector 45 in which is filled NO gas. The measuring tank 43 and the comparison tank 44 are illuminated with light beams of equal intensity from the light source 41. These beams are adsorbed according to the amounts of NO in the channels 20 and 30.
Therefore, by detecting, with the detector 45, the difference in light intensity between the two light beams incident on the detector 45, it is possible to measure the difference of amounts of NO in the channels 20 and 30 to thereby determine the NH.sub.3 gas concentration in the sample gas.
It is well known that combustion exhaust gas usually contains a SO.sub.3 component which exists in the form of sulfur trioxide (SO.sub.3), sulfuric acid mist (H.sub.2 SO.sub.4) and/or sulfates such as ammonium sulfate [(NH.sub.4).sub.2 SO.sub.4 ] and/or acidic ammonium sulfate [(NH.sub.4)HSO.sub.4 ]. Particularly, when the temperature is sufficiently low, sulfur trioxide and sulfuric acid mist react with ammonium according to the following formulas resulting in the production of sulfates. EQU SO.sub.3 +NH.sub.3 +H.sub.2 O.fwdarw.(NH.sub.4)HSO.sub.4, EQU SO.sub.3 +2NH.sub.3 +H.sub.2 O.fwdarw.(NH.sub.4).sub.2 SO.sub.4, EQU H.sub.2 SO.sub.4 +NH.sub.3 .fwdarw.(NH.sub.3)HSO.sub.4, and EQU H.sub.2 SO.sub.4 +2NH.sub.3 .fwdarw.(NH.sub.4).sub.2 SO.sub.4.
The sulfates usually crystallize at a temperature of 250.degree. C. to 200.degree. C. When the ammonia gas analyzer shown in FIG. 1 is used to analyze an exhaust gas containing an SO.sub.3 component, the various tubular members constituting the measuring gas channel 20 and the comparison gas channel 30 tend to be closed by accumulations of crystallized sulfates and various parts thereof tend to be easily corroded by sulfuric acid components causing the ammonia gas analyzer to become inoperative or shortening the useful life thereof.
In order to resolve this difficulty, it may be enough to maintain the temperature of the members constituting the gas channels 20 and 30 at or above 250.degree. C. However, a system for maintaining the temperature at such high temperature is complicated and expensive.